Constantine XI Dragases Palaiologos, Latinized as Palaeologus (Greek: Κωνσταντῖνος ΙΑ' Δραγάσης Παλαιολόγος) was the last reigning Byzantine Emperor, reigning as a member of the Palaiologos dynasty from 1449 to his death in battle at the fall of Constantinople. Following his death, he became a legendary figure in Greek folklore as the "Marble Emperor" who would awaken and recover the Empire and Constantinople from the Ottomans. His death marked the end of the Roman Empire, which had continued in the East for 977 years after the fall of the Western Roman Empire.

Constantine was born in Constantinople, as the eighth of ten children to Manuel II Palaiologos and Helena Dragaš, the daughter of the Serbian magnate Constantine Dragaš. He was extremely fond of his mother and added her surname (Dragases) next to his own dynastic one when he ascended the imperial throne. He spent most of his childhood in Constantinople under the supervision of his parents. He was governor of Selymbria for a time, until surrendering the role to his brother Theodore in 1443. During the absence of his older brother John at the Council of Florence in Italy, Constantine served as his regent in Constantinople (1437–1440).

When his brother, Emperor John VIII Palaiologos, died childless, a dispute erupted between Constantine and his brother Demetrios Palaiologos over the throne. Demetrios drew support by opposing the union of the Orthodox and Catholic churches. The Empress Helena, acting as regent, supported Constantine. They appealed to the Ottoman Sultan Murad II to arbitrate the disagreement.

Murad decided in favor of Constantine, and on 6 January 1449 Constantine was crowned in the cathedral at Mistra by the local bishop.

Sultan Murad died in 1451, succeeded by his 19-year-old son Mehmed II, who was obsessed with the conquest of Constantinople. Desperate for any type of military assistance, Constantine XI appealed to the West, reaffirming the union of Eastern and Roman Churches signed at the Council of Florence, a condition the Catholic Church imposed before any help would be provided. Although some troops did arrive from the mercantile city states in the north of Italy, the Western contribution was negligible compared to the needs, given the Ottoman strength. Constantine also sought assistance from his brothers in Morea, but any help was forestalled by an Ottoman invasion of the peninsula in 1452, executed to tie down the soldiers there.

Constantine led the defence of the city and took an active part in the fighting alongside his troops in the land walls. At the same time, he used his diplomatic skills to maintain the necessary unity between the Genoese, Venetian, and Greek troops.

Constantine died the day the city fell, 29 May 1453. There were no known surviving eyewitnesses to the death of the Emperor and none of his entourage survived to offer any credible account of his death.